5+ Questions

Question 1

Describe the structure of glycogen and explain why it is suitable for energy storage. [ 6 marks]

Answer 1

• Glycogen is a polysaccharide found in animal and bacterial cells.
• It is made from α glucose monomers which are joined by glycosidic
  bonds.
• Glycogen has a compact structure which means more can be stored in a small space.
• Glycogen has a branched structure which
  means it has more ends that can be acted upon simultaneously by enzymes.
• It is therefore rapidly hydrolysed into glucose monomers.
• Glycogen is insoluble so it does not diffuse out of cells and does not draw water into the cells by osmosis.

Question 2

Explain how a one-way flow of blood is maintained through the heart as it travels from the left atrium to the aorta. [5 marks]

Answer 2

• As the left atrium fills with blood from the pulmonary vein, the pressure
  rises.
• When this pressure exceeds that in the left ventricle the atrioventricular valve opens. Blood travels from the atrium into the ventricle.
• The ventricle walls contract increasing the pressure and forcing the atrioventricular valve to close. This prevents the backflow of blood into
  the atrium.
• The ventricle walls contract strongly forcing blood through the semi-lunar valves into the aorta. As the heart relaxes, the blood in the
  aorta is under higher pressure than the left ventricle.
• The semi-lunar valves
  close preventing blood flowing back into the ventricle.

Question 3

The movement of substances across cell membranes is affected by membrane structure. Describe how. [5 marks}

Answer 3

• The phospholipid bilayer of the cell membrane allows lipid-soluble molecules to diffuse through, and prevents the diffusion of polar and lipid-insoluble substances.
• Carrier proteins allow facilitated diffusion, active transport and co-transport.

The shape of the carrier protein determines which substances move, and the number of carriers and membrane surface area determine how much movement.

• Fluidity and permeability of the membrane is also
  affected by cholesterol.

Question 4

Describe the sequence of events involved in transmission across a
cholinergic synapse.

Do not include details on the breakdown of acetylcholine in your answer.

Answer 4

An action potential arrives at the presynaptic membrane and it is depolarised.
This causes calcium channels to open, causing calcium ions to move into the
synaptic knob. This causes synaptic vesicles to fuse with the presynaptic
membrane and release acetylcholine neurotransmitter. Acetylcholine diffuses
across the synaptic cleft and binds to receptors on the postsynaptic
membrane. This causes sodium ions to enter the postsynaptic membrane
leading to depolarisation.